Effect of Slope Stability on Stabilized Soil due to ... · factor of safety static dynamic 100 % bcs 3.07 1.34 bcs + 10% gbs 3.14 1.31 bcs + 1% lime + 10% gbs 3.14 1.54 bcs + 2% lime

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Effect of Slope Stability on Stabilized Soil

due to Earthquake Loads using GeoStudio

Amritha R 1, K V Manjunath 2and Dr R Prabhakara3

1. Graduate student, 2.Associate Professor, and 3. Professor & Head of Department,

Department of Civil Engineering, M.S.Ramaiah Institute of Technology, Bangalore, Karnataka, India

---------------------------------------------------------------------***---------------------------------------------------------------------Abstract – Black cotton soil found in many central

parts of India and Karnataka poses several serious

problems for civil engineers in the buildings, roads,

slopes, retaining structures, etc. This paper evaluates

the possible use of Granulated Blast Furnace Slag (GBS)

to stabilize Black Cotton Soil (BCS). The soil sample

collected from Kadur, Chikmanglur District of

Karnataka was classified as highly compressible

clay(CH) as per Indian Standard Classification System

(ISCS). In this investigation, the response of slopes made

of GBS stabilized BCS under dynamic loading was

studied using finite element method. The slope was

assumed to be constructed with BCS stabilized with

(GBS) along with small amounts of lime viz, 1, 2 and 4

% and for a slope of 15m height. These soil mass along

with the slope was subjected to an amplitude of 0.1g

which was equivalent to highest earthquake in the

region as per past earthquake records. The stability of

slope was analysed and factor of safety was

determined. Factor of safety obtained for unstabilized

soil slope is compared with stabilized soil slope. It was

observed that the stabilized soil performs better under

seismic condition when compared to unstabilized soil

slope.

Key Words: Slope stability, Black cotton Soil, GBS, Factor

of Safety, Dynamic Analysis.

1.INTRODUCTION

In India, expansive soils are usually called Black Cotton

soil (BCS). These soils show high strength under dry

condition (in summer), but undergoes rapid reduction in

strength when it is saturated (during monsoon). The

properties of BCS can be enhanced by blending it with

industrial wastes like Fly Ash, GBS, GGBS, Rice Husk Ash,

etc. GBS, which is a slag material of sand-sized particles, is

one such material which is used to enhance the strength of

BCS. It is not only useful in enhancing the strength of the

soil, but is also a very useful way of disposing industrial

waste and makes the stabilization process more eco-

friendly.

Slopes are one of the main Civil Engineering components

in embankments and earth dams. Stability of these man-

made slopes is of great importance. The stability or

potential to withstand the movement of soil is mainly

measured by its shear strength. There are many causes for

the failure of slopes. Earthquake is one of these main

causes.

Earthquake is one of the most devastating forms of natural

disaster. It causes a lot of damage to life and property. It is

a result of sudden release of energy from earth’s crust

which creates seismic waves. Liquefaction is also one of

the effects of earthquake where in the soil loses its

strength and transforms from solid to liquid state. This

effect leads to sinking of soil and collapse of structures.

Inertia forces which are induced in the slope due to

earthquake results in the displacement of the soil. When

this displacement exceeds certain limit the slope is said to

have failed, because the factor of safety drops below unity

during earthquake. In order to mitigate this failure, it is

tried to stabilize the BCS using GBS and lime so that it may

be used in constructing the slopes which would be more

strong.

2. MATERIALS AND METHOD

2.1 Location of Material.

The BC soil sample used for this study was collected from

Kadur, Chikmaglur District of Karnataka at a depth of 1m

below ground level. The GBS sample used for this study

was procured from Jindal Steel Plant, Bellary in Karnataka.

2.2 Testing method

Properties of BCS and GBS were determined as per Indian

Standard code and are tabulated in Table 1. The soil




obtained from the site was carefully pulverized with

rammer to break lumps. It was then oven dried and mixed

in different proportions with GBS and lime. The different

proportions tested for are tabulated in Table 2. Laboratory

tests were performed on these proportions to determine

their strength parameters.

Table 1: Properties of soil and GBS

Properties BCS GBS

Liquid limit (%) 61 NA

Plastic limit (%) 23.2 NA

Shrinkage limit (%) 14.43 NA-

Specific gravity 2.668 2.525

Maximum Dry Density

(kN/m3)

15.4

13.6

Optimum Moisture Content

(%) 21.5 15.2

CBR value (soaked) (%) 0.11 NA

CBR value (unsoaked) (%) 1.58 NA

28th day UCC strength (kPa) 122 NA

Indian Standard Soil

Classification CH ML

Cohesion (kPa) 90 0

Angle of internal friction () 26 36

Table 2: Combinations of additives with soil

Sl.

No. BCS (%) GBS(%) LIME(%)

1 100 10 0

2 100 10 1

3 100 10 2

4 100 10 4

3. RESULTS AND DISCUSSION

3.1 Compaction properties

Compaction properties for the blended mixes were

determined as per mini compaction test proposed by

Sreedharan and Sivapullaiah (2005). The MDD decreased

on addition of GBS and lime whereas OMC increased for

the same. As lime needs water for chemical reactions and

becomes stronger, the OMC increased because the

presence of optimum content of water for lime provides

more strength.

3.2 Shear strength

Direct shear tests were conducted as per IS 2720 (Part

13) – 1986. These experiments were used to determine

the shear strength of the soil in terms of C and Phi.(Vide

table 3) Samples of various mixes were prepared with

maximum dry density in the laboratory and tested.

Table 3: Direct shear test results

PROPORTIONS DENSITY

in

(kN/m3)

C

in

(kPa)

Phi

in

(degrees)

100 % BCS 15.08 90 25.5

BCS + 10% GBS 14.27 82 28.5

BCS + 1% LIME + 10%

GBS

14.27 76 31

BCS + 2% LIME + 10%

GBS

14.27 70 35

BCS + 4% LIME + 10%

GBS

14.27 63 40.5

4. ANALYSIS OF SLOPE STABILITY

Geo Studio includes elementary features of SLOPE/W,

SEEP/W, SIGMA/W, QUAKE/W, TEMP/W, CTRAN/W,

AIR/W, and VADOSE/W for solving slope stablity and

other related geotechnical analysis. In the present study

two features were primarily used to study the stability,

namely, SLOPE/W which gives the stability of the slope

and QUAKE/W which performs earthquake analysis of the

soil




5. RESULTS AND DISCUSSION

The static and dynamic factors of safety have been given in

Tables 4 to 8. For a slope of 1:1, from Table 4, it was

observed that addition of 10% GBS reduced the dynamic

factor of safety but the addition of 1% Lime increased the

factor of safety and on further addition of lime, the factor

of safety reduced compared to 1% but was found to be

higher than plain BCS. From this, it was understood that

the addition of 10%GBS and 1% lime gives us the highest

dynamic factor of safety. It was also seen that 10 % GBS

and 1 % lime was found to give the highest dynamic factor

of safety only for slope angles of 1:1 and 1:0.9. From Table

6, Table 7 and Table 8, the results show that the factor of

safety increased on addition of lime and it is seen that the

highest factor of safety is obtained for 10% GBS and 4%

lime.

After finalizing the geometry and obtaining material

properties, the slope was modelled in Geostudio SLOPE/W

by keeping the height constant at 15m and the slope

angles were varied using slope angles 45, 48, 51, 55

and 61. Fig 1, shows the analytical model for 45. The

other slope angles were also modelled similarly. SLOPE/W

gives us the static factor of safety of the slope as shown in

Fig 2. Using the same geometry and properties, the same

model was run in QUAKE/W to analyze the same slope for

seismic load. The seismic file is then imported to

SLOPE/W and the dynamic factor of safety was

determined using Newmarks’ Deformation. Fig. 3 shows

the slip surface for the seismic load. Similarly, the slope

and properties were modelled and analysed for all the

proportions shown in Table 3.

Fig -1 Geometry of slope

Fig -2 Critical slip surface of 100%BCS slope for static

load

Fig -3 Critical slip surface of 100%BCS slope for

dynamic load

Table 4: Factor of Safety for slope angle 45

15 m Ht embankment

(1:1) V:H FACTOR OF

SAFETY Static Dynamic

100 % BCS 3.07 1.34

BCS + 10% GBS 3.14 1.31

BCS + 1% LIME + 10% GBS 3.14 1.54

BCS + 2% LIME + 10% GBS 3.15 1.50

BCS + 4% LIME + 10% GBS 3.20 1.47


15 m Ht embankment

(1V : 0.9 H)

FACTOR OF SAFETY

Static Dynamic

100 % BCS 2.30 1.32

BCS + 10% GBS 3.02 1.41

BCS + 1% LIME + 10% GBS 3.10 1.42

BCS + 2% LIME + 10% GBS 3.10 1.38

BCS + 4% LIME + 10% GBS 3.16 1.35

15m Height slope(1:1 – V:H) – 100%BCS –(STATIC FOS =3.071)

15m Height slope(1:1 – V:H) – 100%BCS –(DYNAMIC FOS =1.324)





5 m Ht embankment

(1 V : 0.8 H)

FACTOR OF SAFETY

Static Dynamic

100 % BCS 2.99 1.36

BCS + 10% GBS 2.96 1.33

BCS + 1% LIME + 10% GBS 2.87 1.45

BCS + 2% LIME + 10% GBS 2.93 1.41

BCS + 4% LIME + 10% GBS 2.88 1.52

Table 7: Factor of Safety for 55

15 m Ht embankment

(1 V : 0.7 H)

FACTOR OF SAFETY

Static Dynamic

100 % BCS 2.88 1.51

BCS + 10% GBS 2.88 1.46

BCS + 1% LIME + 10% GBS 2.83 1.48

BCS + 2% LIME + 10% GBS 2.83 1.44

BCS + 4% LIME + 10% GBS 2.80 1.74


15 m Ht embankment

(1V: 0.5H)

FACTOR OF SAFETY

Static Dynamic

100 % BCS 2.75 1.77

BCS + 10% GBS 2.84 1.76

BCS + 1% LIME + 10% GBS 2.81 1.69

BCS + 2% LIME + 10% GBS 2.77 1.69

BCS + 4% LIME + 10% GBS 2.76 1.89

Fig 1: Static FOS for various Slope Angles

Fig 2: Dynamic FOS for various Slope Angles

Fig 3: Dynamic FOS for Varying Proportions

45 48 51 55 63

45 48 51 55 63




6. CONCLUSIONS

The following conclusions was drawn from the results of

the slope stability analyses.

The static factor of safety increases as the

percentage of lime increases, it was seen that for a

slope of 1:1, the static factor safety increased by

2.18% when compared to BCS, on addition of 10%

GBS.

On further addition of 1% lime and 10% GBS, the

static factor of safety was 2.32% higher than that

for BCS.

While for 2% and 4% lime, there was an increase

of 2.57% and 4.23% compared to BCS

respectively.

The dynamic factor of safety for stabilized soil is

higher when lime and GBS were added.

It was observed for a slope of 1:1, the dynamic

factor of safety for 10% GBS decreased by 2.67%

when compared to 100% BCS, while 1% lime

+10% BCS gave an increase of FOS by 14.68%

when compared to 100% BCS.

Also the FOS for 2% lime and 4% lime was found

to be higher by 11.55% and 9.46% respectively

when compared to 100%BCS.

An optimum percentage of lime is obtained as the

slop angle is varied, for slope of 1:1 and 1:0.9, the

optimum percentage of lime was 1% + 10% GBS.

While for the other slopes, the optimum

percentage lime was found to be 4% lime.

Also it was observed that the dynamic factor of

safety increased as the slope angle increased. This

was because as the slope increased, the mass of

the soil in the slope reduced hence there was an

increase of factor of safety.

REFERENCES

1. G Sridevi and A Sreerama Rao(2011),

Utilization Of GBS In Road Sub-Base,

Proceedings of Indian Geotechnical Conference,

December 15-17,2011, Kochi (Paper No. H-

076)

2. Laxmikant Yadu, and R K Tripathi (2013),

“Stabilization Of Soft Soil With Granulated Blast

Furnace Slag And Flyash” ,International Journal

of Research in Engineering and Technology.

ISSN:2319-1163.

3. Huy Thanh Pham , Htet Zaw Oo, Cheng Jing,(

2013 ) “Stability Of Slope And Seepage Analysis In

Earth Dam Using Numerical Finite Element

Model”, Study of Civil Enginreering and

Architecture (SCEA) Volume 2 Issue 4, December

2013.

4. Stability Modelling With SLOPE/W 2007

Version, An Engineering Methodology, Third

Edition, March 2008, GEO-SLOPE International Ltd.

5. Dynamic Modelling With QUAKE/W 2007

Version, An Engineering Methodology, Third

Editioni, March 2008, GEO-SLOPE International

Ltd.

6. Indian Standard Codes IS:2720

BIOGRAPHIES

Name : Amritha R Designation : Graduate student Qualification : B.E.(Civil), M.Tech(Structural Engg) Research Area : Soil Stabilization using GBS

Name : K.V.Manjunath Designation : Associate Professor Qualification : B.E.(Civil), M.Tech(GeotechnicalEngg),(PhD) Research Area : Soil Stabilization using GGBS

Name : Dr R Prabhakara

Designation : Professor & Head

Qualification : M.Tech (Construction

Technology), PhD (Civil Engg)

Research Area : Materials and Structures

Documents

Effect of Slope Stability on Stabilized Soil due to ... · factor of safety static dynamic 100 % bcs 3.07 1.34 bcs + 10% gbs 3.14 1.31 bcs + 1% lime + 10% gbs 3.14 1.54 bcs + 2% lime